Enyne [4 + 4] Photocycloaddition: Bridged 1,2,5-Cyclooctatrienes

Abstract

Enyne photocycloaddition with a 2-pyridone yields a mixture of products including amide-bridged 1,2,5-cyclooctatrienes, the first examples of enyne [4 + 4] adducts. Four regio- and stereochemical isomers of the [4 + 4] adduct are possible. All appear to be too strained to be isolated, but they have been identified as their [2 + 2] cyclobutane dimers. Cyclobutane and cyclobutene adducts have also been isolated, [2 + 2] addition products possibly related to the unstable [4 + 4] adducts via Cope rearrangement. Calculations suggest that [3,3] rearrangements have high energy barriers, however, making thermal interconversion unlikely.

Enyne [4+4] Cycloaddition/Oxidation: Ring Contraction via Cyclopropanones and Their Anionic Ring‐Opening Reactions

“…[1,2] Both inter-a nd intramolecular examples have been reported as well as catalysis by acids and transition metals,a ll of which have been extensively investigated. [3,4] Thecycloadducts formed in these higher-order cycloadditions are incapable of isomerizing to arenes however, and unless an alternative pathway to relieve strain is available,t he products often undergo dimerization reactions that result in complex mixtures. 1!3,S cheme 1).…”

“…Recently we reported that 1,3-enynes can be participants in [4+ +4] cycloadditions and found them to work well with pyridones (4, 7), pyrones and anthracene. [3,4] Thecycloadducts formed in these higher-order cycloadditions are incapable of isomerizing to arenes however, and unless an alternative pathway to relieve strain is available,t he products often undergo dimerization reactions that result in complex mixtures. [5,6] Stability through steric shielding can be designed into strained allenes,asdescribed by Johnson and Price.…”

Formation and isomerization of polycyclic 1,5-enynes

“…2 During our investigations of 2-pyridone photochemistry we have studied its cycloaddition with 1,3-enynes and discovered two different pathways based on the presentation of the enyne. 3,4 When the enyne is attached through the alkyne (1), only [4+4] cyclization results and the highly strained allene product 2 is, in most cases, unstable and undergoes further reaction. In contrast, when the enyne is linked to the pyridone through the alkene (3), only [2+2] products are observed (see Scheme 1).…”

Enyne-2-pyrone [4 + 4]-Photocycloaddition: Sesquiterpene Synthesis and a Low-Temperature Cope Rearrangement

“…25 Cope rearrangement of the 1,2,5-heptatriene unit found in 27 to give a 1,5-enyne (e.g., 24) had been considered in our earlier enyne-pyridone photochemistry studies but was believed to be an unlikely pathway. 11 Huntsman demonstrated that the parent 1-hexen-5-yne underwent Cope rearrangement to give 1,2,5-hexatriene at 340°C. 26 Computational studies suggest that these pathways do not require radical intermediates.…”